Deep beneath the fields between France and Switzerland, inside a 27-kilometre ring where protons race at nearly the speed of light, something strange is happening.
For 50 years, the Standard Model has successfully described nearly every particle and force that scientists have measured. However, in April 2026, two major particle experiments at CERN found the same problem: a rare type of particle decay is behaving differently than the equations predict.
The difference is small but persistent. The LHCb experiment measured a deviation of four standard deviations from what the Standard Model says should happen. In particle physics, this means there is only a one-in-16,000 chance that this result is random error. It is not yet a discovery, but it cannot be easily ignored.
The decay is extremely rare. Scientists found it by studying approximately 650 billion particle collisions recorded between 2011 and 2018. Physicists call this type of process a "penguin" decay, named after a bet made in the late 1970s.
Even more interesting, another detector called CMS found the same problem using different data. Two independent teams and two different machines produced the same unexpected result.
Scientists have several explanations. Some believe it involves complex mathematical calculations that are difficult to solve. Others suggest completely new particles may exist, such as a Z-prime boson or leptoquarks.
The discovery threshold requires five sigma. With only four sigma, the result is intriguing but not yet confirmed. However, LHCb has collected three times more data since 2018, and future upgrades will provide even more information. The answer may soon appear in the data being collected beneath the Alps.